High-frequency furnace for heating metal. HDTV installation - operating principle for hardening. High-frequency furnaces for hardening from JSC “SMK”

A furnace for melting metals at home can also be used to quickly heat metal elements, for example, when tinning or forming them. The operating characteristics of the presented installations can be adjusted to a specific task by changing the parameters of the inductor and the output signal of the generating sets - this way you can achieve their maximum efficiency.

Hardening of steels with high frequency currents (HFC) is one of the most common methods of surface heat treatment, which allows increasing the hardness of the surface of workpieces. Used for parts made of carbon and structural steels or cast iron. Induction hardening HDTV is one of the most economical and technologically advanced methods of hardening. It makes it possible to harden the entire surface of a part or its individual elements or zones that experience the main load.

In this case, under the hardened hard outer surface of the workpiece, unhardened viscous layers of metal remain. This structure reduces fragility, increases the durability and reliability of the entire product, and also reduces energy consumption for heating the entire part.

High frequency hardening technology

HDTV surface hardening is a heat treatment process to increase the strength characteristics and hardness of the workpiece.

The main stages of surface hardening of HDTV are induction heating to a high temperature, holding at it, then rapid cooling. Heating during hardening of HDTV is carried out using a special induction installation. Cooling is carried out in a bath with a coolant (water, oil or emulsion) or by spraying it onto the part from special shower installations.

Temperature selection

For the correct completion of the hardening process, the correct selection of temperature, which depends on the material used, is very important.

Steels based on carbon content are divided into hypoeutectoid - less than 0.8% and hypereutectoid - more than 0.8%. Steel with carbon less than 0.4% is not hardened due to the resulting low hardness. Hypoeutectoid steels are heated slightly above the temperature of the phase transformation of pearlite and ferrite to austenite. This occurs in the range of 800-850°C. Then the workpiece is quickly cooled. When cooled sharply, austenite transforms into martensite, which has high hardness and strength. A short holding time makes it possible to obtain fine-grained austenite and fine-needle martensite; the grains do not have time to grow and remain small. This steel structure has high hardness and at the same time low brittleness.

Hypereutectoid steels are heated slightly lower than hypoeutectoid steels, to a temperature of 750-800°C, that is, incomplete hardening is performed. This is due to the fact that when heated to this temperature, in addition to the formation of austenite, a small amount of cementite, which has a higher hardness than martensite, remains undissolved in the metal melt. After rapid cooling, austenite transforms into martensite, and cementite remains in the form of small inclusions. Also in this zone, carbon that has not had time to completely dissolve forms solid carbides.

In the transition zone during high-frequency quenching, the temperature is close to the transition temperature, and austenite with ferrite residues is formed. But, since the transition zone does not cool down as quickly as the surface, but cools down slowly, as during normalization. At the same time, the structure in this zone improves, it becomes fine-grained and uniform.

Overheating the surface of the workpiece promotes the growth of austenite crystals, which has a detrimental effect on brittleness. Underheating prevents the complete ferrite-perrite structure from transforming into austenite, and unhardened spots may form.

After cooling, high compressive stresses remain on the metal surface, which increase the performance properties of the part. Internal stresses between the surface layer and the middle must be eliminated. This is done using low-temperature tempering - holding at a temperature of about 200°C in an oven. To avoid the appearance of microcracks on the surface, it is necessary to minimize the time between hardening and tempering.

You can also carry out the so-called self-tempering - cool the part not completely, but to a temperature of 200 ° C, while heat will remain in its core. Then the part should cool slowly. This will equalize internal stresses.

Induction installation

The HDTV induction heat treatment unit is a high-frequency generator and inductor for HDTV hardening. The part to be hardened can be located in or near the inductor. The inductor is made in the form of a coil, with a copper tube wound on it. It can have any shape depending on the shape and size of the part. When alternating current passes through the inductor, an alternating electromagnetic field appears in it, passing through the part. This electromagnetic field causes eddy currents known as Foucault currents to occur in the workpiece. Such eddy currents, passing through layers of metal, heat it to a high temperature.

A distinctive feature of induction heating using HDTV is the passage of eddy currents on the surface of the heated part. This way, only the outer layer of the metal is heated, and the higher the frequency of the current, the smaller the depth of heating, and, accordingly, the depth of hardening of the high-frequency frequency. This makes it possible to harden only the surface of the workpiece, leaving the inner layer soft and tough to avoid excessive brittleness. Moreover, you can adjust the depth of the hardened layer by changing the current parameters.

The increased frequency of the current allows you to concentrate a large amount of heat in a small area, which increases the heating rate to several hundred degrees per second. Such a high heating rate moves the phase transition to a higher temperature zone. In this case, the hardness increases by 2-4 units, up to 58-62 HRC, which cannot be achieved with volumetric hardening.

For the correct implementation of the HDTV hardening process, it is necessary to ensure that the same clearance is maintained between the inductor and the workpiece over the entire hardening surface, and mutual touching must be avoided. This is ensured, if possible, by rotating the workpiece in the centers, which allows for uniform heating, and, as a consequence, the same structure and hardness of the surface of the hardened workpiece.

The inductor for hardening HDTV has several versions:

• single- or multi-turn annular - for heating the outer or inner surface of parts in the form of bodies of revolution - shafts, wheels or holes in them;
• loop - for heating the working plane of the product, for example, the surface of the bed or the working edge of the tool;
• shaped - for heating parts of complex or irregular shape, for example, gear teeth.

Depending on the shape, size and depth of the hardening layer, the following HDTV hardening modes are used:

• simultaneous - the entire surface of the workpiece or a certain zone is heated at once, then also cooled simultaneously;
• continuous-sequential - one zone of a part is heated, then when the inductor or part is displaced, another zone is heated, while the previous one is cooled.

Simultaneous high-frequency heating of the entire surface requires large amounts of power, so it is more profitable to use it for hardening small parts - rolls, bushings, pins, as well as part elements - holes, necks, etc. After heating, the part is completely lowered into a tank with coolant or sprayed with a stream of water.

Continuous-sequential hardening of high-frequency particles allows you to harden large-sized parts, for example, the crowns of gear wheels, since during this process a small zone of the part is heated, which requires less power of the high-frequency generator.

Cooling parts

Cooling is the second important stage of the hardening process; the quality and hardness of the entire surface depends on its speed and uniformity. Cooling occurs in coolant tanks or by spray. For high-quality hardening, it is necessary to maintain a stable temperature of the coolant and prevent it from overheating. The holes in the sprayer must be of the same diameter and spaced evenly, this way the same metal structure on the surface is achieved.

To prevent the inductor from overheating during operation, water is constantly circulated through the copper tube. Some inductors are made combined with a workpiece cooling system. Holes are cut in the inductor tube through which cold water enters the hot part and cools it.

Advantages and disadvantages

Hardening of parts using HDTV has both advantages and disadvantages. The advantages include the following:

• After high-frequency quenching, the part retains a soft center, which significantly increases its resistance to plastic deformation.
• The cost-effectiveness of the process of hardening HDTV parts is due to the fact that only the surface or zone that needs to be hardened is heated, and not the entire part.
• During mass production of parts, it is necessary to set up the process and then it will be automatically repeated, ensuring required quality hardening
• The ability to accurately calculate and adjust the depth of the hardened layer.
• The continuous-sequential hardening method allows the use of low-power equipment.
• Short heating and holding time high temperature contributes to the absence of oxidation, decarburization of the top layer and the formation of scale on the surface of the part.
• Rapid heating and cooling does not result in large warpage and distortion, which allows for a reduction in finishing allowance.

But it is economically feasible to use induction installations only for mass production, and for single production, purchasing or manufacturing an inductor is unprofitable. For some parts with complex shapes, induction production is very difficult or impossible to obtain a uniform hardened layer. In such cases, other types of surface hardening are used, for example, gas-flame or volumetric hardening.

Induction heating is a method of non-contact heating with high frequency currents (RFH - radio-frequency heating, heating by radio frequency waves) of electrically conductive materials.

Description of the method.

Induction heating is the heating of materials electric currents, which are induced by an alternating magnetic field. Consequently, this is the heating of products made of conductive materials (conductors) by the magnetic field of inductors (sources of alternating magnetic field). Induction heating is carried out as follows. An electrically conductive (metal, graphite) workpiece is placed in a so-called inductor, which is one or several turns of wire (most often copper). Powerful currents of various frequencies (from tens of Hz to several MHz) are induced in the inductor using a special generator, as a result of which an electromagnetic field appears around the inductor. The electromagnetic field induces eddy currents in the workpiece. Eddy currents heat the workpiece under the influence of Joule heat (see Joule-Lenz law).

The inductor-blank system is a coreless transformer in which the inductor is the primary winding. The workpiece is the secondary winding, short-circuited. The magnetic flux between the windings is closed through the air.

At high frequencies, eddy currents are displaced by the magnetic field they themselves generate into thin surface layers of the workpiece Δ ​​(Surface effect), as a result of which their density increases sharply, and the workpiece heats up. The underlying layers of metal are heated due to thermal conductivity. It is not the current that is important, but the high current density. In the skin layer Δ, the current density decreases by e times relative to the current density on the surface of the workpiece, while 86.4% of the heat is released in the skin layer (of the total heat release. The depth of the skin layer depends on the radiation frequency: the higher the frequency, the thinner skin layer It also depends on the relative magnetic permeability μ of the workpiece material.

For iron, cobalt, nickel and magnetic alloys at temperatures below the Curie point, μ has a value from several hundred to tens of thousands. For other materials (melts, non-ferrous metals, liquid low-melting eutectics, graphite, electrolytes, electrically conductive ceramics, etc.) μ is approximately equal to unity.

For example, at a frequency of 2 MHz, the skin depth for copper is about 0.25 mm, for iron ≈ 0.001 mm.

The inductor becomes very hot during operation, as it absorbs its own radiation. Moreover, it absorbs thermal radiation from a hot workpiece. Inductors are made from copper tubes cooled by water. Water is supplied by suction - this ensures safety in case of burnout or other depressurization of the inductor.

Application:
Ultra-clean non-contact melting, soldering and welding of metal.
Obtaining prototypes of alloys.
Bending and heat treatment of machine parts.
Jewelry making.
Processing of small parts that can be damaged by gas flame or arc heating.
Surface hardening.
Hardening and heat treatment of parts with complex shapes.
Disinfection of medical instruments.

Advantages.

High-speed heating or melting of any electrically conductive material.

Heating is possible in a protective gas atmosphere, in an oxidizing (or reducing) environment, in a non-conducting liquid, or in a vacuum.

Heating through the walls of a protective chamber made of glass, cement, plastics, wood - these materials absorb electromagnetic radiation very weakly and remain cold during operation of the installation. Only electrically conductive material is heated - metal (including molten), carbon, conductive ceramics, electrolytes, liquid metals, etc.

Due to the MHD forces that arise, intensive mixing of the liquid metal occurs, up to keeping it suspended in air or a protective gas - this is how ultra-pure alloys are obtained in small quantities (levitation melting, melting in an electromagnetic crucible).

Since heating is carried out through electromagnetic radiation, there is no contamination of the workpiece with torch combustion products in the case of gas-flame heating, or with the electrode material in the case of arc heating. Placing samples in an inert gas atmosphere and high heating rates will eliminate scaling.

Ease of use due to the small size of the inductor.

The inductor can be made of a special shape - this will allow it to be evenly heated over the entire surface of parts of a complex configuration, without leading to their warping or local non-heating.

It is easy to carry out local and selective heating.

Since the most intense heating occurs in the thin upper layers of the workpiece, and the underlying layers are heated more gently due to thermal conductivity, the method is ideal for surface hardening of parts (the core remains viscous).

Easy automation of equipment - heating and cooling cycles, temperature adjustment and maintenance, feeding and removal of workpieces.

Induction heating units:

For installations with an operating frequency of up to 300 kHz, inverters based on IGBT assemblies or MOSFET transistors are used. Such installations are designed for heating large parts. To heat small parts, high frequencies are used (up to 5 MHz, medium and short waves), high-frequency installations are built on vacuum tubes.

Also, to heat small parts, high-frequency installations are being built using MOSFET transistors for operating frequencies up to 1.7 MHz. Controlling transistors and protecting them at higher frequencies presents certain difficulties, so higher frequency settings are still quite expensive.

The inductor for heating small parts is small in size and has low inductance, which leads to a decrease in the quality factor of the working oscillatory circuit at low frequencies and a decrease in efficiency, and also poses a danger to the master oscillator (the quality factor of the oscillatory circuit is proportional to L/C, an oscillatory circuit with a low quality factor is too good “pumped” with energy, forms a short circuit in the inductor and disables the master oscillator). To increase the quality factor of the oscillatory circuit, two ways are used:
- increasing the operating frequency, which leads to more complex and expensive installations;
- use of ferromagnetic inserts in the inductor; pasting the inductor with panels made of ferromagnetic material.

Since the inductor operates most efficiently at high frequencies, induction heating received industrial application after the development and start of production of high-power generator lamps. Before World War I, induction heating had limited use. High-frequency machine generators (works by V.P. Vologdin) or spark-discharge installations were then used as generators.

The generator circuit can, in principle, be anything (multivibrator, RC generator, generator with independent excitation, various relaxation generators), operating on a load in the form of an inductor coil and having sufficient power. It is also necessary that the oscillation frequency be high enough.

For example, to “cut” a steel wire with a diameter of 4 mm in a few seconds, an oscillatory power of at least 2 kW is required at a frequency of at least 300 kHz.

The scheme is selected according to the following criteria: reliability; vibration stability; stability of the power released in the workpiece; ease of manufacture; ease of setup; minimum number of parts to reduce cost; the use of parts that together result in a reduction in weight and dimensions, etc.

For many decades, an inductive three-point generator (Hartley generator, autotransformer generator) was used as a generator of high-frequency oscillations. feedback, circuit based on an inductive loop voltage divider). This is a self-exciting parallel power supply circuit for the anode and a frequency-selective circuit made on an oscillating circuit. It has been successfully used and continues to be used in laboratories, jewelry workshops, industrial enterprises, as well as in amateur practice. For example, during the Second World War, such installations were used surface hardening rollers of the T-34 tank.

Disadvantages of three points:

Low efficiency (less than 40% when using a lamp).

A strong frequency deviation at the time of heating of workpieces made of magnetic materials above the Curie point (≈700C) (μ changes), which changes the depth of the skin layer and unpredictably changes the heat treatment mode. When heat treating critical parts, this may be unacceptable. Also, powerful HDTV installations must operate in a narrow range of frequencies permitted by Rossvyazohrankultura, since with poor shielding they are actually radio transmitters and can interfere with television and radio broadcasting, coastal and rescue services.

When changing workpieces (for example, from a smaller one to a larger one), the inductance of the inductor-workpiece system changes, which also leads to a change in the frequency and depth of the skin layer.

When changing single-turn inductors to multi-turn ones, to larger or smaller ones, the frequency also changes.

Under the leadership of Babat, Lozinsky and other scientists, two- and three-circuit generator circuits were developed that have a higher efficiency (up to 70%) and also better maintain the operating frequency. The principle of their operation is as follows. Due to the use of coupled circuits and weakening of the connection between them, a change in the inductance of the operating circuit does not entail a strong change in the frequency of the frequency-setting circuit. Radio transmitters are designed using the same principle.

Modern HDTV generators are inverters based on IGBT assemblies or powerful MOSFET transistors, usually made according to a bridge or half-bridge circuit. Operate at frequencies up to 500 kHz. The transistor gates are opened using a microcontroller control system. The control system, depending on the task at hand, allows you to automatically hold

A) constant frequency
b) constant power released in the workpiece
c) the highest possible efficiency.

For example, when a magnetic material is heated above the Curie point, the thickness of the skin layer increases sharply, the current density drops, and the workpiece begins to heat up worse. The magnetic properties of the material also disappear and the process of magnetization reversal stops - the workpiece begins to heat up worse, the load resistance decreases abruptly - this can lead to “spreading” of the generator and its failure. The control system monitors the transition through the Curie point and automatically increases the frequency when the load abruptly decreases (or reduces power).

Notes.

If possible, the inductor should be located as close to the workpiece as possible. This not only increases the electromagnetic field density near the workpiece (proportional to the square of the distance), but also increases the power factor Cos(φ).

Increasing the frequency sharply reduces the power factor (proportional to the cube of the frequency).

When heating magnetic materials, additional heat is also released due to magnetization reversal; heating them to the Curie point is much more efficient.

When calculating an inductor, it is necessary to take into account the inductance of the buses leading to the inductor, which can be much greater than the inductance of the inductor itself (if the inductor is made in the form of one turn of small diameter or even part of a turn - an arc).

There are two cases of resonance in oscillatory circuits: voltage resonance and current resonance.
Parallel oscillatory circuit – current resonance.
In this case, the voltage on the coil and on the capacitor is the same as that of the generator. At resonance, the circuit resistance between the branching points becomes maximum, and the current (I total) through the load resistance Rн will be minimal (the current inside the circuit I-1l and I-2s is greater than the generator current).

Ideally, the loop impedance is infinity—the circuit draws no current from the source. When the generator frequency changes in any direction from the resonant frequency, the circuit impedance decreases and the line current (I total) increases.

Series oscillatory circuit – voltage resonance.

The main feature of a series resonant circuit is that its impedance is minimal at resonance. (ZL + ZC – minimum). When tuning the frequency above or below the resonant frequency, the impedance increases.
Conclusion:
In a parallel circuit at resonance, the current through the circuit terminals is 0 and the voltage is maximum.
In a series circuit, on the contrary, the voltage tends to zero and the current is maximum.

The article was taken from the website http://dic.academic.ru/ and revised into a text that is more understandable for the reader by Prominductor LLC.